1. Field of the Invention
The present invention relates to a linear actuator comprising a cabinet, a reversible electric motor with a motor shaft, a reduction gear operatively connected with the motor shaft, a spindle operatively connected with the reduction gear, a spindle nut secured against rotation on the spindle, an activating element connected with the spindle nut and for attachment to the structure in which the actuator is to be incorporated, a rear attachment disposed in extension of the spindle opposite the activating element likewise for attachment of the actuator in the structure in which the actuator is to be incorporated.
2. The Prior Art
State of the art actuators are known e.g. from EP 531 247 A1, EP 586 326 A1, EP 647 799 and EP 662 573 A1 as well as WO 98/30816, all to Linak A/S. These actuators are used inter alia for incorporation in hospital beds and sickbeds. For an example of a hospital bed, reference may be made to EP 488 552 A1 to Huntleigh Technology plc. For hospital beds, the power supply is typically based on a transformer and a rectifier supplemented with a rechargeable battery pack, so that the bed may also be operated even though it is not connected to the mains, e.g. when it is driven from one location to another or during a temporary stay e.g. in an examination room or X-ray room. In emergency situations where the bed must be capable of quickly assuming a specific position, e.g. Trendelenburg's position, the actuators for hospital beds are provided with a disengagement device, where the spindle is disengaged from gear and motor so that e.g. the back and leg sections seek toward the horizontal by their own weight. The adjustment is then made manually by grasping the section and placing it in the desired position. The disengagement device may also be used in the event that the battery pack should fail, or another defect should occur. Where sickbeds and beds for domestic use are involved, equipment such as battery pack and disengagement device adds considerably to the costs. Instead of the rechargeable battery pack, an ordinary small 9V battery may be used, with the drawbacks this entails, and, moreover, it is frequently arranged at a location which is difficult to reach.
In bed structures having integrated spindles for adjustment of the bed, cf. e.g. U.S. Pat. No. 5,269,031 to Alexander, it is known to use a hand crank for manual adjustment of the bed in case of power failure, or altogether carry out the adjustment with a hand crank.
U.S. Pat. No. 2,541,529 to McVicker discloses a single example of an actuator where, under normal conditions, the spindle is driven by an electric motor via a worm gear, which, in emergency situations, can be driven by a hand crank, as the worm wheel is disengaged from the worm. The worm wheel is connected with the spindle via a specially configured, longitudinally slidable, spring-loaded coupling member. For introduction of the hand crank, a cap must first be screwed off, and then the hand crank is introduced and a cap with a bearing, mounted on the hand crank, is screwed on. The structure is bulky, as it requires a great mounting length as well as a large diameter, since the end of the hand crank must be capable of being introduced therein. In addition, the actuator has to be secured in another manner than by a rear attachment, as it must be possible to screw the caps off and on.
The object of the invention is to provide a state of the art actuator which may be operated manually by a hand crank without intervention in its basic structure, and which is additionally constructionally simple and inexpensive to manufacture.